The present invention relates to an induction heating coil, provided in an induction heating apparatus, for heating an object to be heated having low resistance and low magnetic permeability, such as aluminum, in particular.
Iron pans having high magnetic permeability have been used conventionally as pans for use in induction heating cooking apparatuses; however, in recent years, there is an increasing demand for use of pans other than iron pans, such as copper and aluminum pans. In the case that a copper pan or an aluminum pan is induction heated, a high-frequency current having a frequency of approximately 40 to approximately 100 kHz, higher than a frequency of approximately 20 to approximately 30 kHz suited for an iron pan, is required to flow through the heating coil since copper and aluminum have low resistivity and low magnetic permeability. However, when the frequency becomes higher, the high-frequency current flows only near the surface of a conductor owing to the skin effect. In other words, the current does not flow in the central portion of the sectional area; this is the same that the sectional area of the conductor is small in effect, whereby the effective resistance of the heating coil wire itself increases significantly during operation. Hence, as a method for increasing the surface area and thereby effectively decreasing the resistance, a method has been used wherein several to several tens of wires, each wire being a thin conductor (for example, a copper wire of 0.1 mm in diameter) having an insulation covering on its surface, are bundled to produce a heating coil.
In addition, the coil wire serving as a conductor for the heating coil has a multi-stage collective structure wherein stranded wires obtained by bundling wires are further twisted to decrease the effective resistance (effective resistance) developed in effect owing to the skin effect. Furthermore, it is proposed that the stranded wire obtained at least in the first stage is formed by weaving to suppress the increase in the resistance owing to the proximity effect (hereinafter referred to as the proximity action), or the like. The proximity action is a phenomenon wherein when high-frequency currents flow in adjacent conductors, the currents are affected mutually via magnetic fields, and deflection occurs in current distribution, whereby the effective resistances on the surfaces of the conductors increase. The proximity action becomes more significant as the directions of the high-frequency currents are more uniform among the conductors and as the clearances among the conductors are smaller. In the above-mentioned conventional configurations, the resistance of the heating coil with respect to the high-frequency current can be decreased to some extent. However, for further improvement in the efficiency of induction heating, it is necessary to further decrease the resistance of the coil.
In the above-mentioned conventional configurations, the coil resistance (high-frequency resistance) of the heating coil with respect to the high-frequency current can be decreased. However, in the case that a heating coil for heating a pan made of copper or aluminum is produced by adopting these configurations, it is necessary that the conductor diameter of a wire is made as small as possible to 0.1 mm or less and that as many of such wires as possible (for example, 1000 to 2000 wires) are bundled to form a coil wire. When the wire diameter is small as described above, problems occur; for example, the insulator on the surface of the wire is damaged and the conductor is liable to break.
Furthermore, when a terminal for external connection (for connection to an inverter for supplying a high-frequency current to this heating coil) is attached to an end of the coil wire configured as described above, the insulation covering of each wire at the end portion of the coil wire, approximately 10 mm in length, is dipped in a predetermined chemical and eliminated so that its conductor (for example, a copper wire) is exposed. Then, the wires are cleaned and bundled again and then crimped and soldered to the terminal, whereby electrical connection is carried out among the respective conductors mutually and to the terminal.
Hence, a problem of making the work for connecting the terminals for external connection to the ends of the coil wire very complicated is caused.